Fuel cell comprising a composite membrane

ABSTRACT

A fuel cell comprising at least one composite membrane which includes a porous support impregnated with ion exchange resin to make the pores of the support occlusive, wherein the thickness of the composite membrane is less than 0.025 mm. The membrane is strong and has good ionic conductivity. Methods of making the composite membrane are also disclosed.

FIELD OF THE INVENTION

[0001] An integral composite membrane having a thickness of less thanabout 1 mil (0.025 mm) is provided which is useful in electrolyticprocesses and other chemical separations.

BACKGROUND OF THE INVENTION

[0002] Ion exchange membranes (IEM) are used in fuel cells as solidelectrolytes. A membrane is located between the cathode and anode andtransports protons formed near the catalyst at the hydrogen electrode tothe oxygen electrode thereby allowing a current to be drawn from thecell. These membranes are particularly advantageous as they replaceheated acidic liquid electrolytes such as phosphoric acid fuel cellswhich are very hazardous.

[0003] Ion exchange membranes are used in chloralkali applications toseparate brine mixtures and form chlorine gas and sodium hydroxide. Themembrane selectively transports the sodium ions across the membranewhile rejecting the chloride ions. IEM's are also useful in the area ofdiffusion dialysis where for example, caustic solutions are stripped oftheir impurities. The membranes are also useful for pervaporation andvapor permeation separations due to their ability to transfer polarspecies at a faster rate than their ability to transfer non-polarspecies.

[0004] These membranes must have sufficient strength to be useful intheir various applications. Often this need for increased strengthrequires the membranes to be made thicker which decreases their ionicconductivity. For example, ion exchange membranes that are notreinforced such as those commercially available from E. I. DuPont deNemours, Inc. and sold under the trademark Nafion are inherently weak atsmall thicknesses (e.g., less than 0.050 mm) and must be reinforced withadditional materials causing the final product to have increasedthickness. Moreover, these materials cannot be reliably manufacturedpinhole free.

[0005] U.S. Pat. No. 3,692,569 to Grot relates to the use of a coatingof a copolymer of fluorinated ethylene and a sulfonyl-containingfluorinated vinyl monomer on a fluorocarbon polymer that was previouslynon-wettable. The fluorocarbon polymer may include tetrafluoroethylenepolymers (not porous expanded PTFE). This coating provides a topicaltreatment to the surface so as to decrease the surface tension of thefluorocarbon polymer. U.S. Pat. No. 4,453,991 to Grot relates to aprocess for making a liquid composition of a perfluorinated polymerhaving sulfonic acid or sulfonate groups in a liquid medium that iscontacted with a mixture of water and a second liquid such as a loweralcohol. The liquid made by the process may be used as a coating, a castfilm, and as a repair for perfluorinated ion exchange films andmembranes. Cast or coated products made with the liquid composition hadthicknesses on the order of 5 mils (0.125 mm).

[0006] U.S. Pat. No. 4,902,308 to Mallouk, et al. relates to a film ofporous expanded PTFE having surfaces, both exterior and internalsurfaces adjacent to pores, coated with a metal salt of perfluoro-cationexchange polymer. The base film of porous, expanded PTFE had a thicknessof between 1 mil and 6 mils (0.02-0.150 mm). The final composite producthad a thickness of at least 1 mil (0.025 mm) and preferably had athickness of between 1.7 and 3 mils (0.043-0.075 mm). The compositeproduct was permeable to air and the air flow as measured by the Gurleydensometer ASTM D726-58 was found to be between 12 and 22 seconds.

[0007] U.S. Pat. No. 4,954,388 to Mallouk, et al. relates to anabrasion-resistant, tear resistant, multi-layer composite membranehaving a film of continuous perfluoro ion exchange polymer attached to areinforcing fabric by means of an interlayer of porous expanded PTFE. Acoating of a perfluoro ion exchange resin was present on at least aportion of the internal and external surfaces of the fabric and porousePTFE. The composite membrane made in accordance with the teachings ofthis patent resulted in thicknesses of greater than 1 mil (0.025 mm)even when the interlayer of porous ePTFE had a thickness of less than 1mil (0.025 mm).

[0008] U.S. Pat. No. 5,082,472 to Mallouk, et al. relates to a compositemembrane of microporous film in laminar contact with a continuousperfluoro ion exchange resin layer wherein both layers have similar areadimensions. Surfaces of internal pores of ePTFE may be coated at leastin part with perfluoro ion exchange resin coating. The membrane of ePTFEhad a thickness of about 2 mils (0.050 mm) or less and the perfluoro ionexchange layer in its original state had a thickness of about 1 mil(0.025 mm). The ePTFE layer of this composite membrane impartedmechanical strength to the composite structure and the pores of theePTFE were preferably essentially unfilled so as to not block the flowof fluids.

[0009] U.S. Pat. Nos. 5,094,895 and 5,183,545 to Branca, et al. relateto a composite porous liquid-permeable article having multiple layers ofporous ePTFE bonded together and having interior and exterior surfacescoated with a perfluoro ion exchange polymer. This composite porousarticle is particularly useful as a diaphragm in electrolytic cells. Thecomposite articles are described to be relatively thick, preferablybetween from 0.76 and 5 mm.

[0010] U.S. Pat. No. 4,341,615 to Bachot, et al. relates to afluorinated resin base material treated with a copolymer of anunsaturated carboxylic acid and a non-ionic unsaturated monomer for useas a porous diaphragm in the electrolysis of alkaline metal chlorides.The fluorinated resin base material may be reinforced with fibers suchas asbestos, glass, quartz, zirconia, carbon, polypropylene,polyethylene, and fluorinated polyhalovinylidene (col. 2, lines 13-17).Only 0.1 to 6 percent of the total pore volume of the support sheet isoccupied by the carboxylic copolymer.

[0011] U.S. Pat. No. 4,604,170 to Miyake et al. relates to amulti-layered diaphragm for electrolysis comprising a porous layer of afluorine-containing polymer having a thickness of from 0.03 to 0.4 mmwith its interior and anode-side surface being hydrophilic and an ionexchange layer on its cathode surface with the ion exchange layer beingthinner than the porous layer but of at least 0.005 mm and the totalthickness of the diaphragm being from 0.035 to 0.50 mm.

[0012] U.S. Pat. No. 4,865,925 to Ludwig, et al. relates to a gaspermeable electrode for electrochemical systems. The electrode includesa membrane located between and in contact with an anode and a cathode.The membrane, which may be made of expanded polytetrafluoroethylene, maybe treated with an ion exchange membrane material with the resultingmembrane maintaining its permeability to gas. Membrane thicknesses aredescribed to be between 1 and 10 mils, (0.025-0.25 mm), with thicknessesof less than 5 mils (0.125) to be desirable. Examples show that membranethicknesses range from 15 to 21 mils.

[0013] Japanese Patent Application No. 62-240627 relates to a coated oran impregnated membrane formed with a perfluoro type ion exchange resinand porous PTFE film to form an integral unit. No water or surfactantwere used in the manufacture of this membrane. The combination isaccomplished by fusion or by coating and does not provide for permanentadhesion of the ion exchange resin to the inside surface of the PTFEfilm. The weight ratio of the perfluoro ion exchange resin to PTFE isdescribed to be in the range of 3 to 90% with a preferable weight ratioof 10 to 30%.

[0014] Japanese Application No. 62-280230 and 62-280231 relate to acomposite structure in which a scrim or open fabric is heat laminatedand encapsulated between a continuous perfluoro ion exchange membraneand an ePTFE sheet thus imparting tear strength to the structure. Thecomposite membrane was not used for ionic conduction.

[0015] Additional research has also been conducted on the use ofperfluorosulfonic acid polymers with membranes of expanded porouspolytetrafluoroethylene such as that described in Journal Electrochem.Soc., Vol. 132, No. 2, February 1985, p. 514-515. The perfluoro ionexchange material was in an ethanol based solvent without the presenceof water or surfactant. Moreover, ultrasonic energy in the treatment ofthis membrane.

[0016] Heretofore and as represented by the references discussed above,there is a need for an integral ultra-thin strong ion exchange compositemembrane, with long term chemical and mechanical stability that has athickness before swelling of at most 1 mil (0.025 mm), with more than90% of the pore volume of the membrane filled with a perfluoro ionomerto render it at least substantially occlusive and that is capable ofswelling without deterioration of mechanical properties.

SUMMARY OF THE INVENTION

[0017] An ultra-thin integral composite membrane is provided including aporous polymeric membrane having a structure of micropores of polymerwith a porosity of greater than 35%, an average pore diameter of lessthan 10 microns and a thickness of at most 0.025 mm and a perfluoro ionexchange polymer impregnated within the micropores so as to render themicropores substantially occlusive, wherein the composite membrane isimpermeable to gases and liquids and is substantially free of pinholes.Porous polymeric membranes suitable for this invention include membranesmade of perfluoroalkyloxy resin, fluorinated ethylene propylene,polyolefins, polyamides, cellulosics, polycarbonates, fluorinated andchlorinated polymers, and polysulfones. Perfluoro ion exchange materialssuitable for use with this invention include perfluorinated sulfonicacid resin, perfluorinated carboxylic acid resin, polyvinyl alcohol,divinyl benzene, and styrene based polymers. A reinforcement backing mayalso be provided.

[0018] Methods for making the ultra-thin integral composite membranesare also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a schematic cross-section of the composite membrane thatis fully impregnated with the ion exchange material.

[0020]FIG. 2 is a schematic cross-section of the composite membrane thatis fully impregnated with the ion exchange material and has a backingmaterial attached thereto.

[0021]FIG. 3 is a photomicrograph of a cross-section of expanded PTFEthat has not been treated with any ion exchange material at amagnification of 2.5 kX.

[0022]FIG. 4 is a photomicrograph of a cross-section of expanded PTFEfully impregnated with an ion exchange material at a magnification of5.1 kX.

DETAILED DESCRIPTION OF THE INVENTION

[0023] An ultra-thin composite membrane is provided and includes a basematerial of microporous membrane with a thickness less than 1 mil (0.025mm) having a microstructure of micropores and perfluoro ion exchangeresin that substantially impregnates the microporous membrane so as toocclude the micropores. The ultra-thin composite membrane may beemployed in many different types of applications including for example,chemical separation, electrolysis in fuel cells and batteries,pervaporation, gas separation, dialysis separation, industrialelectrochemistry such as chlor-alkali, and other electrochemicaldevices, catalysis as a super acid catalyst and use as a catalystsupport in enzyme immobilization.

[0024] The ultra-thin composite membrane is mechanically strong and issubstantially and uniformly pore occlusive so that it is particularlyuseful as an ion exchange material. Ultra-thin is hereby defined as 1mil (0.025 mm) or less. Uniform is hereby defined as continuousimpregnation with the ion exchange material so that no pin holes orother discontinuities exist within the composite structure. In addition,pore occlusive is hereby defined as pores being substantiallyimpregnated (i.e., at least 90%) with the perfluoro ion exchangematerial rendering the final material air impermeable with a Gurleynumber of infinity.

[0025] The microporous membrane which serves as the base material forthe composite has a porosity of greater than 35% and preferably has aporosity of between 70-95%. The pores of the microstructure have adiameter less than 10 μm, are preferably between 0.05 and 5 μm, and aremost preferably about 0.2 μm. The thickness of the membrane is at most 1mil (0.025 mm) preferably between 0.06 mils (0.19 μm) and 0.8 mils (0.02mm), and most preferably between 0.50 mils (0.013 mm) and 0.75 mils(0.019 mm). Materials from which this microporous membrane can be madeinclude for example, perfluoroalkyloxy (PFA), fluorinated ethylenepropylene (FEP), polyolefins, polyamides, cellulosics, polycarbonates,fluorinated and/or chlorinated polymers, and polysulfones. A mostpreferred material is expanded porous polytetrafluoroethylene (PTFE)made in accordance with the teachings of U.S. Pat. No. 3,593,566 hereinincorporated by reference. This material is commercially available in avariety of forms from W. L. Gore & Associates, In., of Elkton, Md.,under the trademark GORE-TEX®. The expanded PTFE membrane can be made ina number of thickness ranging from 0.00025 inches to 0.125 inches (6 μmto 3 mm) with the preferred thickness for the present invention being atmost 1 mil (0.025 mm) and most preferably between 0.50 mils (0.013 mm)and 0.75 mils (0.019 mm). The expanded PTFE membrane can be made withporosities ranging from 20% to 98%, with the preferred porosity for thepresent invention being 70-95%. FIG. 3 shows a photomicrograph of theinternal microstructure of expanded PTFE used as the base material.

[0026] An ion exchange material dissolved in a solvent and mixed with asurfactant is uniformly applied so as to impregnate and occlude themicropores of the base material. Suitable ion exchange materials includefor example, perfluorinated sulfonic acid resin, perfluorinatedcarboxylic acid resin, polyvinyl alcohol, divinyl benzene, styrene-basedpolymers and metal salts with or without a polymer. A mixture of theseion exchange materials may also be employed in treating a membrane.Solvents that are suitable for use with the ion exchange materialinclude for example, alcohols, carbonates, THF (tetrahydrofuran), water,and combinations thereof.

[0027] A surfactant having a molecular weight of greater than 100 mustbe employed with the ion exchange material to ensure impregnation of thepores. Surfactants or surface active agents having a hydrophobic portionand a hydrophilic portion may be utilized. Preferable surfactants arethose having a molecular weight of greater than 100 and may beclassified as anionic, nonionic, or amphoteric which may be hydrocarbonor fluorocarbon-based and include for example, Merpol®, a hydrocarbonbased surfactant or Zonyl®, a fluorocarbon based surfactant, bothcommercially available from E. I. DuPont de Nemours, Inc. of Wilmington,Del.

[0028] A most preferred surfactant is a nonionic material, octylphenoxypolyethoxyethanol having a chemical structure:

[0029] where x=10 (average)

[0030] known as Triton® X100, commercially available from Rohm & Haas ofPhiladelphia, Pa.

[0031]FIG. 1 shows a schematic view of the composite membrane with ionexchange material 2 and the base material 4 so that the micropores ofthe interior of the base material 4 are fully impregnated. The finalcomposite membrane has a uniform thickness free of any discontinuitiesor pinholes on the surface. The micropores of the membrane are 100%occluded thus causing the composite membrane to be impermeable toliquids and gases FIG. 4 shows a scanning electron photomicrograph ofthis composite membrane.

[0032] Alternatively, the ion exchange material and surfactant mixture 2may be applied to the membrane 4 so that most of the pores are uniformlytreated rendering the membrane substantially impregnated with the ionexchange material. The composite membrane is still free of anydiscontinuities or pinholes.

[0033] Optionally, and as shown schematically in FIG. 2, the compositemembrane may be reinforced with a woven or non-woven material 6 bondedto one side of the membrane. Suitable woven materials include forexample, scrims made of woven fibers of expanded porouspolytetrafluoroethylene, commercially available from W. L. Gore &Associates, Inc. of Elkton, Md.; webs made of extruded or orientedpolypropylene netting commercially available from Conwed, Inc. ofMinneapolis, Minn.; and woven materials of polypropylene and polyesterof Tetko Inc., of Briarcliff Manor, N.Y. Suitable non-woven materialsinclude for example, a spun-bonded polypropylene commercially availablefrom Reemay Inc. of Old Hickory, Tenn.

[0034] The treated membrane may then be further processed to remove thesurfactant with the use of various low molecular weight alcohols. Thisis accomplished by soaking or submerging the membrane in a solution of,for example, water, isopropyl alcohol, methanol and/or glycerin. Duringthis step, the surfactant which was originally mixed in solution withthe perfluoro ion exchange material is removed. Slight swelling of themembrane occurs. The perfluoro ion exchange material remains within thepores of the base material as it is not soluble in the lower molecularweight alcohol.

[0035] The membrane is further treated by boiling in a suitable swellingagent, preferably water causing it to then slightly swell in the x and ydirection. Additional swelling occurs in the z-direction. A compositemembrane results having a higher ion transport rate (at least 20 timeshigher than in its unswollen state) that is also strong. The swollenmembrane retains its mechanical integrity unlike the membranesconsisting only of the perfluoro ion exchange material andsimultaneously maintains desired ionic transport characteristics. Acorrelation exists between the content of the swelling agent within themembrane structure and transport properties of the membrane. A swollenmembrane will transport chemical species faster than an unswollenmembrane.

[0036] Although the membrane has excellent long term chemical stability,it can be susceptible to poison by organics. The organics can be removedby regeneration in which the membrane is boiled in a strong acid such asnitric or chromic acid.

[0037] To prepare the inventive membrane, a support structure such as apolypropylene woven fabric may first be laminated to the untreated basemembrane by any conventional technique including hot roll lamination,ultrasonic lamination, adhesive lamination, forced hot air lamination solong as the technique does not damage the integrity of the membrane. Asolution is prepared containing a perfluorosulfonic acid resin insolvent mixed with one or more surfactants. The solution may be appliedto the membrane by any conventional coating technique includingforwarding roll coating, reverse roll coating, gravure coating, doctorcoating, kiss coating, as well as dipping, brushing, painting, andspraying so long as the liquid solution is able to penetrate theinterstices and micropores of the base material. Excess solution fromthe surface of the membrane may be removed. The treated membrane is thenimmediately placed into an oven to dry. Oven temperatures may range from60-200° C., preferably 120-160° C. so as to lock the perfluoro ionpolymer inside the membrane and prevent it from migrating to the surfaceduring drying. This step may be repeated until the membrane becomescompletely transparent. Typically between 2 to 8 treatments are requiredbut the actual number of treatments is dependent on the surfactantconcentration and thickness of the membrane. If the membrane is preparedwithout a support structure, both sides of the membrane may be treatedsimultaneously thereby reducing the number of treatments required.

[0038] The oven treated membrane is then soaked in a low molecularweight alcohol, as described above to remove the surfactant. Themembrane is then boiled as described above in a swelling agent underpressure ranging from 0 to 20 atmospheres absolute thereby increasingthe amount of swelling agent the treated membrane is capable of holding.

[0039] Alternatively, the ion exchange material may be applied to themembrane without the use of a surfactant. This procedure requiresadditional exposure to the perfluoro ion exchange resin but does notthen need to be soaked in alcohol. A vacuum may also be used to draw theion exchange material into the membrane.

[0040] Another alternative to the process of preparing the inventivemembrane involves the selection and use of a surfactant having low watersolubility with the perfluoro ion solution. Surfactants with low watersolubility include Zonyl® FSO, a fluorocarbon based surfactantcommercially available from E. I. DuPont de Nemours, Inc. By using thistype of surfactant, the heat treatment step may be eliminated. Theresulting ion exchange treated membrane made by this process may be usedfor different aqueous applications and other chemical environmentswithout any effect due to the surfactant.

[0041] Because the base membrane is exceptionally thin (at most 1 mil)(0.025 mm) with the resulting composite membrane being very thin andonly marginally distorted in the x and y directions, it is able toselectively transport ions at a faster rate than heretofore has beenachieved with only a slight lowering of the selectivity characteristicsof the membrane.

[0042] The following testing procedures were employed on the samplesprepared as described in the examples described below.

Test Procedures

[0043] Air Permeability—Gurley Number Method

[0044] The resistance of samples to air flow was measured by a Gurleydensometer (ASTM D726-58) manufactured by W. & L. E. Gurley & Sons. Theresults were reported in terms of Gurley Number defined as the time inseconds for 100 cubic centimeters of air to pass through 1 square inch(6.45 sq. cm.) of a test sample at a pressure drop of 4.88 inches (12.4cm.) of water.

[0045] Strength Modulus

[0046] Strength testing was carried out on an Instron Model 1122.Samples were one inch wide. Gauge length (distance between clamps) wastwo inches (5.08 cm.). Samples were pulled at a rate of 500% per minute.The cross head speed was 20 inches per minute.

[0047] Thickness

[0048] Thickness of the base material was determined with the use of asnap gauge (Johannes Kafer Co. Model No. F1000/302). Measurements weretaken in at least four areas of each specimen. Thickness of the driedcomposite membrane were also obtained with the use of the snap gauge.Thicknesses of swollen samples were not measurable due to thecompression or residual water on the surface of the swollen membranewith the snap gauge. Thickness measurements of the swollen membrane werealso not able to be obtained with the use of scanning electronmicroscopy due to interferences with the swelling agents.

[0049] Moisture Vapor Transmission Rate (MVTR)

[0050] Potassium Acetate Method

[0051] Moisture vapor transmission rates were determined by thefollowing procedure. Approximately 70 ml. of a solution consisting of 35parts by weight of potassium acetate and 15 parts by weight of distilledwater was placed into a 133 ml. polypropylene cup, having an insidediameter of 6.5 cm. at its mouth. An expanded polytetrafluoroethylene(PTFE) membrane having a minimum MVTR of approximately 85,000 g/m²-24 hras tested by the method described in U.S. Pat. No. 4,862,730 to Crosbyand available from W. L. Gore & Associates, Inc. of Newark, Del., washeat sealed to the lip of the cup to create a taut, leakproof,microporous barrier containing the solution.

[0052] A similar expanded PTFE membrane was mounted to the surface of awater bath. The water bath assembly was controlled at 23° C. plus orminus 0.2° C., utilizing a temperature controlled room and a watercirculating bath.

[0053] The sample to be tested was allowed to condition at a temperatureof 23° C. and a relative humidity of 50% prior to performing the testprocedure. Samples were placed so the microporous polymeric membrane wasin contact with the expanded polytetrafluoroethylene membrane mounted tothe surface of the water bath and allowed to equilibrate for at least 15minutes prior to the introduction of the cup assembly.

[0054] The cup assembly was weighed to the nearest {fraction (1/1000)}g. and was placed in an inverted manner onto the center of the testsample.

[0055] Water transport was provided by the driving force between thewater in the water bath and the saturated salt solution providing waterflux by diffusion in that direction. The sample was tested for 10minutes and the cup assembly was then removed, weighed again within{fraction (1/1000)} g.

[0056] The MVTR of the sample was calculated from the weight gain of thecup assembly and was expressed in grams of water per square meter ofsample surface area per 24 hours.

[0057] Peel Strength

[0058] Peel strength or membrane adhesion strength tests were conductedon samples prepared with reinforced backings. Test samples were preparedhaving dimensions of 3 inch by 3.5 inch (7.62 cm×8.89 cm). A 4 inch by 4inch (10.2 cm×10.2 cm) chrome steel plate with an Instron tensile testmachine Model No. 1000 were also used. Double coated vinyl tape(3M-#419) 1 inch (2.54 cm) wide was placed over the edges of the chromeplate so that tape covered all edges of the plate.

[0059] The sample material was then mounted on top of the adhesiveexposed side of the tape and pressure was applied so that sample wassecured.

[0060] The plate and sample were then installed in the Instron in ahorizontal position. The upper crosshead was lowered so that the jaws ofthe machine closed flat and tightened upon the sample. The uppercrosshead was then slowly raised pulling the layer of perfluoro ionmaterial from the reinforced backing. When the composite membrane hadbeen detached from the reinforced backing, the test was complete.Adhesion strength was estimated from the average strength needed to pullthe composite membrane from the reinforced backing.

[0061] Electrical Conductance

[0062] The electrical conductance in the Z-direction (otherwise known asthrough conductance) was measured. A sample of swollen compositemembrane (cut to a 1 inch diameter circle) was placed between two 0.680inch (1.73 cm) diameter copper contacts. A 5 lb. (2268 g) weight wasplaced above the top contact. The contacts were connected to a HewlettPackard Model 3478A multimeter. The resistance was then read. Prior tothis measurement, the thickness of the dried preswollen compositemembrane was determined as described above. Conductance was calculatedaccording to the formula:

C=1/R

[0063] wherein

[0064] R=resistance measured in ohms

[0065] C=conductance measured in mhos

[0066] Ionic Conduction Rate

[0067] The conductivity of the composite membrane was tested to measurethe ionic conduction rate in terms of micromhos per minute. This testwas performed with two 900 ml. compartments between which the treatedmembrane was placed. The exposed surface area of membrane was 7.07 sqin. (45.65 sq cm). One compartment (the retentate side) was filled with1 M NaCl solution. The other side (the permeate side) was filled withpure distilled water. Both compartments were stirred continuously and atthe same speed with two electric mixers using polypropylene impellors.The conductivity of the permeate side was recorded every 5 minutes foran hour with a hand-held conductivity meter, Omega Model No. PHH80. Thetotal ionic conduction rate was determined by taking the average slopeof a graph of conductivity over time.

[0068] Linear Expansion Tests

[0069] The percentage swelling in the x- and y-directions weredetermined. The length and width of the composite membranes were firstmeasured with a Mitutoyo Model #505-627-50 caliper prior to boiling andswelling. Final measurements were taken after the samples were boiledand swelled. Percent linear expansion were then calculated for both thex- and y-directions.

[0070] Weight Change

[0071] The percent weight change on samples was also prepared. Herecomposite membranes were weighed prior to boiling and swelling and thenafter swelling. All weight measurements were taken with Mettler Balance,Model No. AT400. The percent weight change was then calculated.

EXAMPLE 1

[0072] A sample of expanded polytetrafluoroethylene membrane made inaccordance with the teachings of U.S. Pat. No. 3,593,566, hereinincorporated by reference. The membrane, with a nominal thickness of0.75 mils (0.02 mm) and a 0.2 micrometer pore size, was mounted on a 6inch wooden embroidery hoop. A solution was prepared comprising 95% byvolume of a perfluorosulfonic acid/tetra-fluoroethylene copolymer resin(in H+ form) in a solution of low molecular weight alcohols comprisingpropanol, butanol, and methanol known as Nafion NR-50 (1100 EW)commercially available from E. I. DuPont de Nemours, Inc. and 5% of anonionic surfactant of octyl phenoxy poly ethoxyethanol known as TritonX100 commercially available from Rohm & Haas of Philadelphia, Pa. Thissolution was brushed on both sides of the membrane so as to impregnateand substantially occlude the micropore structure. The sample was thendried in the oven at 140° C. for 30 seconds. The procedure was repeatedtwo more times to fully occlude the micropores. The sample was thensoaked in isopropanol for 5 minutes to remove the surfactant. Afterrinsing with distilled water and allowing to dry at room temperature, afinal coat of the Nafion-surfactant solution as described above wasapplied. The wet membrane was again dried in the oven at 140° C. for 30seconds and soaked in isopropanol for 2 minutes. The membrane wasfinally boiled in distilled water for 10 minutes under atmosphericpressure to swell the treated membrane. Gurley numbers for this materialare summarized in Table 3. Ionic conductive rates are summarized inTable 4. The strength modulus may be found in Table 5; percent linearexpansion may be found in Table 6; and percent weight change of thissample may be found in Table 7. The swollen membrane was later dried toa dehydrolyzed state in an oven at 140° C. for 30 seconds. The thicknessof the dried composite membrane was measured and found to beapproximately the same thickness as the base material.

EXAMPLE 2

[0073] A sample of expanded porous PTFE membrane made in accordance withthe teachings of U.S. Pat. No. 3,593,566 having a pore size of 0.2micrometers and nominal thickness of 0.75 mils (0.02 mm) and a GurleyDensometer air flow of 24 seconds was placed on top of a netting ofpolypropylene obtained from Conwed Plastics Corp. of Minneapolis, Minn.The two materials were bonded together on a laminator with 10 psigpressure, a speed of 15 feet per minute and a temperature of 200° C. Noadhesives were used. The reinforced membrane sample was then placed on a6 inch wooden embroidery hoop. A solution of 96% by volume of aperfluorosulfonic acid TFE copolymer resin in alcohol Nafion NR-50 (1100EW) commercially available from E. I. DuPont de Nemours, Inc. and 4% ofthe nonionic surfactant Triton X-100 obtained from Rohm & Haas wasprepared. This solution was brushed on the membrane side only tosubstantially occlude the micropores and the sample was dried in an ovenat 130° C. This procedure was repeated three more times to fully occludethe micropores. The sample was then baked in an oven at 140° C. for 5minutes. The sample was soaked in isopropanol for 5 minutes to removethe surfactant. The membrane was then boiled in distilled water for 30minutes under atmospheric pressure causing the treated membrane toswell. Gurley numbers for this material are summarized in Table 3.

[0074] This sample was tested for its peel strength in accordance withthe method described above. The linear bond strength was found to be2.06 lb./sq. in. (1450 kg/m²).

EXAMPLE 3

[0075] A sample of expanded porous polytetrafluoroethylene membrane madein accordance with the teachings of U.S. Pat. No. 3,593,566, having athickness of 0.5 mils (0.01 mm) with a pore size of 0.2 micrometer wasmounted on a 6 inch wooden embroidery hoop. A solution of 100% Nafionresin solution, perfluorosulfonic acid/TFE copolymer resin in a solventmixture of propanol, butanol, and methanol known commercially from E. I.DuPont de Nemours, Inc. as Nafion® solution NR-50 (1100 EW) without theaddition of any surfactants was brushed onto both sides of the membraneto substantially occlude the micropores. The sample was then placed inan oven at 160° C. to dry. This procedure was repeated four more timesuntil the membrane was completely transparent and the micropores werefully occluded. The sample was then boiled in distilled water for 30minutes at atmospheric pressure causing the membrane to swell. Gurleynumbers for this material are summarized in Table 3. The electricalconductivity was measured and summarized in Table 2.

EXAMPLE 4

[0076] A sample of expanded porous polytetrafluoroethylene membrane madein accordance with the teachings of U.S. Pat. No. 3,593,566 having athickness of 0.5 mils (0.01 mm) and a pore size 0.2 micrometers wasmounted on a 6 inch wooden embroidery hoop. A solution of 99% by volumeNafion NR-50 commercially available from E. I. DuPont de Nemours, Inc.and 1% surfactant mixture was prepared. The surfactant mixture consistedof 50% of a nonionic surfactant, Triton X-100 commercially availablefrom Rohm & Haas Corp. and 50% Zonyl FSO commercially available from E.I. DuPont de Nemours, Inc. This solution was brushed on both sides ofthe membrane and was allowed to dry at room temperature. This procedurewas repeated 4 more times until the sample was completely transparentand to fully occlude the micropores. The sample was not treated so as toremove the surfactant. The composite membrane was boiled in distilledwater for 5 minutes causing the membrane to swell. The Gurley number forthis material is summarized in Table 3.

EXAMPLE 5

[0077] A sample of expanded porous polytetrafluoroethylene membrane madein accordance with the teachings of U.S. Pat. No. 3,593,566, having athickness of 0.5 mils (0.01 mm) with a pore size of 0.2 micrometer wasmounted onto a 6 inch wooden embroidery hoop. A solution of 95% byvolume Nafion NR-50 (1100 EW) commercially available from E. I. DuPontde Nemours, Inc. and 5% of a nonionic surfactant, Triton X-100commercially available from Rohm & Haas was prepared. The solution wasbrushed on both sides of the membrane with a foam brush and the excesswas wiped off. The wet membrane was dried in an oven at 140° C. for 30seconds. Three additional coats of solution were applied to the membranein the same manner to fully occlude the micropores. The membrane wasthen soaked in isopropanol for 2 minutes to remove the surfactant. Themembrane was rinsed with distilled water and allowed to dry at roomtemperature. A final treatment of the Nafion-Triton solution wasapplied. The wet membrane was dried in the oven at 140° C. for 30seconds, then soaked in isopropanol for 2 minutes. Finally, the membranewas boiled in distilled water for 5 minutes. Moisture vapor transmissionrates for the treated membrane were measured and are summarized inTable 1. The Gurley number of the treated membrane are summarized inTable 3.

EXAMPLE 6

[0078] A sample of expanded porous polytetrafluoroethylene membrane madein accordance with the teachings of U.S. Pat. No. 53,593,566, having anominal thickness of 0.75 mils (0.02 mm) and a pore size of 0.2micrometers was mounted onto a 6 inch wooden embroidery hoop. The GurleyDensometer air flow on this membrane was 2-4 seconds. A solution of 95%by volume Nafion NR-50 (1100 EW) commercially available from E. I.DuPont de Nemours, Inc. and 5% Triton X-100 non-ionic surfactant fromRohm & Haas was prepared. The solution was brushed on both sides of themembrane with a foam brush and the excess was wiped off. The wetmembrane was dried in the oven at 140° C. for 30 seconds. Threeadditional coats of solution were applied in the same manner. Themembrane was then soaked in isopropanol for 2 minutes. After rinsingwith distilled water and allowing to dry at room temperature, a finalcoat of the Nafion surfactant solution was applied. The wet membrane wasdried in the oven at 140° C. for 30 seconds, then soaked in isopropanolfor 2 minutes. This material was not boiled. No swelling other than theminor swelling during the surfactant removal occurred. The ionicconduction rate for this material is presented in Table 4.

EXAMPLE 7

[0079] A sample of expanded porous polytetrafluoroethylene membrane madein accordance with the teachings of U.S. Pat. No. 3,593,566, having anominal thickness of 0.75 mils (0.02 mm) and a pore size of 0.2micrometers was mounted onto a 5 inch plastic embroidery hoop. TheGurley Densometer air flow on this membrane was 24 seconds. A solutionof 95% by volume Nafion NR-50 (1100 EW) commercially available from E.I. DuPont de Nemours, Inc. and 5% Triton X-100 non-ionic surfactant fromRohm & Haas was prepared. The solution was brushed on both sides of themembrane with a foam brush and the excess was wiped off. The wetmembrane was dried in the oven at 140° C. for 30 seconds. Two additionalcoats of solution were applied in the same manner so as to fully occludethe micropores. The membrane was then soaked in isopropanol for 2minutes. After rinsing with distilled water and allowing to dry at roomtemperature, a final coat of the same Nafion NR-50 Triton X-100 solutionwas applied. The wet membrane was dried in the oven at 140° C. for 30seconds and then soaked in isopropanol for 2 minutes to remove thesurfactant. The sample was rinsed and dried at room temperature. Noboiling occurred.

[0080] This sample was weighed before it was mounted on the 5 inchplastic hoop. Following treatment, it was removed from the hoop andweighed again. The ion exchange polymer content was directly calculatedby determining the weight change before and after treatment. The ionexchange content for this sample was found to be 98.4 mg or 9.81 gramsper square meter of membrane. A sample of Nafion 115 (5 mils, 0.13 mm)commercially available from E. I. DuPont de Nemours, Inc. was cut to a 1inch (25.4 mm) by 1 inch (25.4 mm) sample, weighed and found to be 216grams per square meter.

[0081] Nafion Comparative Samples

[0082] Nafion 117, a perfluorosulfonic acid cation exchange membrane,unreinforced film of 1100 equivalent weight commercially available fromE. I. DuPont de Nemours Co., Inc., having a quoted nominal thickness of7 mils (0.18 mm) was obtained. The samples, originally in the hydratedswollen state were measured in the x- and y-directions and weighed. Thesamples were then dried at room temperature to an unswollen state andthen remeasured from which expansion and weight change measurementsfound in Tables 6 and 7 were calculated. Nafion 115, a perfluorosulfonicacid cation exchange membrane, unreinforced film of 1100 equivalentweight also commercially available from E. I. DuPont de Nemours, Inc.,having a nominal thickness of 5 mils (0.1 mm) was obtained. This samplewas also obtained commercially in the hydrated swollen state. TABLE 1Moisture Vapor Transmission Rates (MVTR) Sample ID* MVTR (grams/m²-24hrs. 5 25,040 Nafion 117 23,608

[0083] TABLE 2 Electrical Conductivity Sample ID* Conductivity(micromhos) 3 1,277 Nafion 117 1,214

[0084] TABLE 3 Gurley Numbers Final Swollen Sample Thickness BaseMaterial Membrane ID (mm)* Gurley No. (sec) Gurley Number (sec) 1 0.022-4 Total occlusion 2 0.02 2-4 Total occlusion 3 0.01 2-4 Totalocclusion 4 0.01 2-4 Total occlusion 5 0.01 2-4 Total occlusion

[0085] TABLE 4 Ionic Conduction Rate Ionic Conduction Rate Sample ID(micromhos/minute) 1 119 (swollen) 6 5.1 (unswollen) Nafion 115 15.9(swollen)

[0086] TABLE 5 Strength Modulus Thickness Strength Modulus Sample ID(mm) (lb per dry sq. in.)** 1  0.02* 15150 Nafion 115 0.13 12750

[0087] TABLE 6 Percent Linear Expansion Average Average Un- % Un- %swollen Swollen Expansion swollen Swollen Expansion Sample (x) (x) in(y) (y) in ID (mm) (mm) x-direction (mm) (mm) y-direction 1 124.4 124.4— 123.3 123.3 — Nafion 125.5 137.7 +9.7 127.3 149.7 +17 117

[0088] TABLE 7 Percent Weight Change Unswollen Swollen % weight SampleID wt (g) wt (wet) (g) change (wet) 1 0.2515 1.0273 +308% Nafion 1175.5700 7.5106  +35%

1. A fuel cell comprising at least one integral air impermeablecomposite membrane comprising: at least one polymeric support having amicrostructure of micropores, said microstructure defining a porosity inthe range of about 70% to 98% within said polymeric support, at leastone ion exchange resin filling said microstructure such that saidcomposite membrane is air impermeable, said composite membrane having athickness of at most 0.8 mils and an ionic conduction rate of at least5.1 μmhos/min.
 2. The fuel cell according to claim 1, wherein thepolymeric support is expanded polytetrafluoroethylene.
 3. The fuel cellaccording to claim 1, wherein the thickness of the composite membrane is0.06 mils to 0.8 mils.
 4. The fuel cell according to claim 1, whereinthe thickness of the composite membrane is 0.5 mils to 0.8 mils.
 5. Thefuel cell according to claim 1, wherein the thickness of the compositemembrane is at most 0.5 mils.
 6. The fuel cell according to claim 1,wherein the ion exchange resin comprises perfluorinated sulfonic acidresin.
 7. The fuel cell of claim 1, further comprising a reinforcementbacking bonded to a side thereof.
 8. The fuel cell of claim 1, whereinthe thickness of the composite membrane is at most 0.5 mils, wherein thepolymeric support is expanded polytetrafluoroethylene, and wherein theion exchange resin comprises perfluorinated sulfonic acid resin.
 9. Thefuel cell of claim 1, wherein the microstructure includes nodesinterconnected with fibrils.
 10. A fuel cell comprising a substantiallyair occlusive integral composite membrane having a polymeric supportwith a microstructure of pores, said microstructure filled with an ionexchange resin, said composite membrane has an ionic conduction rate ofat least 5.1 μmhos/min, said composite membrane prepared by, (a)providing a polymeric support having a microstructure of micropores; (b)applying an ion exchange resin solution to said polymeric support; and(c) repeating step (b) until said micropores are sufficiently filledwith ion exchange resin to form an air occlusive integral compositemembrane.
 11. The fuel cell of claim 10, wherein said step (b) furtherincludes, (b1) drying said support after each application of ionexchange resin solution to remove solvent from said solution.
 12. Thefuel cell according to claim 10, wherein said support comprises expandedpolytetrafluoroethylene.
 13. The fuel cell according to claim 10,wherein the composite membrane has a thickness of 0.06 to 0.8 mils. 14.The fuel cell according to claim 10, wherein the composite membrane hasa thickness of 0.5 to 0.8 mils.
 15. The fuel cell according to claim 10,wherein the composite membrane has a thickness of at most 0.8 mils. 16.The fuel cell according to claim 10, wherein the composite membrane hasa thickness of at most 0.5 mils.
 17. The fuel cell according to claim10, wherein the ion exchange resin is a perfluorinated sulfonic acidresin.
 18. The fuel cell according to claim 17, wherein the polymericsupport is expanded polytetrafluoroethylene.
 19. The fuel cell accordingto claim 18, wherein the composite membrane has a thickness of at most0.8 mils.
 20. A fuel cell comprising an integral air impermeablecomposite membrane comprising: a polymeric support having amicrostructure of micropores, said microstructure defining a porosity inthe range of about 70% to 98% within said support, at least one ionexchange resin filling said microstructure such that said compositemembrane is air impermeable, said composite membrane having a thicknessof at most 0.8 mils.
 21. The fuel cell of claim 20, wherein thethickness of said composite membrane is in the range of between 0.06 and0.8 mils.
 22. The fuel cell of claim 20, wherein the thickness of saidcomposite membrane is in the range of between about 0.5 and 0.8 mils.23. The fuel cell of claim 20, wherein the thickness of said compositemembrane is at most 0.5 mils.
 24. The fuel cell of claim 20, wherein thepolymeric support is expanded polytetrafluoroethylene.
 25. The fuel cellof claim 20, wherein the polymeric support is expandedpolytetrafluoroethylene and the ion exchange resin is a perfluorinatedsulfonic acid resin.
 26. The fuel cell of claim 20, wherein saidcomposite membrane is prepared by, (a) providing a polymeric supporthaving a microstructure of micropores; (b) applying an ion exchangeresin solution to said polymeric support; and (c) repeating step (b)until said micropores are sufficiently filled with ion exchange resin toform an air impermeable composite membrane.
 27. The fuel cell of claim20, wherein the composite membrane is heated at 60° C. to 200° C. 28.The fuel cell of claim 20, wherein the composite membrane is heated at120° C. to 160° C.
 29. The fuel cell of claim 28, wherein the thicknessof said composite membrane is in the range of between about 0.5 and 0.8mils, wherein the polymeric support is expanded polytetrafluoroethylene,wherein the ion exchange resin is a perfluorinated sulfonic acid resin,and wherein said composite membrane is prepared by, (a) providing apolymeric support having a microstructure of micropores; (b) applying anion exchange resin solution to said polymeric support; and (c) repeatingstep (b) until said micropores are sufficiently filled with ion exchangeresin to form an air impermeable composite membrane.
 30. A fuel cellconsisting essentially of a composite membrane consisting essentiallyof: a support having a microstructure of micropores, said microstructuredefining a porosity in the range of about 20% to 98% within saidsupport, at least one ion exchange resin filling said microstructuresuch that said composite membrane is air impermeable, said compositemembrane having a thickness of at most 0.8 mils, said composite membranebeing heated to 60° C. to 200° C., and said ion exchange resin being aperfluorinated sulfonic acid resin.
 31. The fuel cell of claim 30,wherein said support has a pore diameter of less than 10 microns. 32.The fuel cell of claim 30, wherein the thickness is at most 0.5 mils.33. The fuel cell of claim 30, wherein the support is a polymericsupport.
 34. The fuel cell of claim 35, wherein the membrane is heatedat 120° C. to 160° C.
 35. A fuel cell comprising a composite membraneconsisting essentially of: a support of expanded polytetrafluoroethylenehaving a microstructure of micropores, said microstructure defining aporosity in the range of about 70% to 98% within said support, at leastone perfluorinated sulfonic acid ion exchange resin filling at least 90%of said microstructure, said composite membrane having a thickness of atmost 0.8 mils, said composite membrane being heated to 60° C. to 200° C.36. A fuel cell according to claim 35, wherein the pore diameter is lessthan 10 microns.
 37. A fuel cell according to claim 35, wherein the porediameter is between 0.05 microns and 5 microns.
 38. A fuel cellaccording to claim 35, wherein the composite membrane is heated to 120°to 160° C.
 39. A fuel cell according to claim 35, wherein the microporesare fully occluded.
 40. A fuel cell according to claim 35, wherein areinforcement backing is bonded to the membrane.